OPERCULAR VALVES. 51 



Opercular Valves. — These are situated on each side of 

 the slit or orifice leading into the sack; from their shape, 

 their powers of movement, their separation by flexible 

 membrane from the shell, to which they serve as a lid, 

 they appear at first as if they constituted an element very 

 distinct from the shell itself, but this is not the case. They 

 are, together with the opercular membrane, as essentially as 

 the whole of what is externally visible, a part of the modified 

 carapace, of which they occupy the upper or posterior extre- 

 mity: from tracing the metamorphoses, or even by comparison 

 of a Balanus with Pollicipes, there can be no doubt of the 

 truth of this conclusion. The opercular valves are four in 

 number, — a pair of scuta and a pair of terga ; but the latter 

 in Coronula diadema and regince, are either aborted or re- 

 presented by a mere rudiment ; and in Xenobalanus both 

 scuta and terga are quite absent. In several cases, more 

 especially in the genus Pyrgoma (PL 13, fig. 1 b), the 

 scutum and tergum on each side are calcified together, so 

 that sometimes not even a trace of the line of junction can 

 be discovered. In most cases the scutum is firmly united, 

 being articulated in a manner presently to be described, to 

 the tergum ; but in Coronula, Tubicinella (PI. 17, fig. 3 c), 

 and Platylepas, the ends of these valves are simply approxi- 

 mated. 



Scuta. — These valves are important, inasmuch as the 

 animal's body is attached to them; in PI. 25, fig. 1, the 

 broken line, surrounding a, b, shows where the body has 

 been cut, in removing the scutum on the near side, the 

 other scutum, S, being left articulated to the tergum, T. 

 In shape the scuta are generally sub-triangular ; but in some 

 species of Pyrgoma and in Chelonobia, &c. they are much 

 elongated. The lines of growth are usually prominent ; and 

 along the occludent margins the alternate, or sometimes every 

 third or fourth line, is developed into a knob, which pro- 

 duces a serrated edge, serving to lock the two opposed valves 

 together ; there is, however, no trace of this structure in 

 Coronula and Tubicinella. In some species of Pyrgoma, a 

 ledge of considerable breadth (PI. 13, fig. 3 e, &c.) is de- 

 veloped along the occludent margins of the two scuta, as 

 well as of the two terga, giving them an anomalous struc- 



